1 Field of the Invention
The present invention relates to a semiconductor light emitting device, more particularly to a semiconductor laser of double hetero structure and of stripe geometry, comprising a first semiconductor layer of an active region and second and third layers adjacent to the first semiconductor layer having a greater energy gap and a smaller index of refraction (of light) than those of the first layer. Such a semiconductor laser is used, for example, for a light source for a long distance and a high bit rate light transmission system using a single mode optical fiber. Gallium-aluminium-arsenic/gallium-arsenic-(Ga.sub.1-2 Al.sub.x As/GaAs) and indium-gallium-arsenic-phosphorus/indium-phosphorus (In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y /InP) are used for the material of such a semiconductor laser.
2. Description of the Prior Art
A prior art structure of such a semiconductor laser is illustrated in FIGS. 1A and 1B. The semiconductor laser S' comprises an n type semiconductor substrate 1' of InP, an n type clad (cladding) layer 2' of InP, an active layer 3' of In.sub.1-x Ga.sub.x -As.sub.y P.sub.1-y, the constitution parameters x and y being, for example, approximately 0.7 and 0.6, respectively, for the wavelength 1.3 .mu.m of the light, a P type clad layer 4' of InP, and a P type ohmic contact layer 6' of In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y. The clad layers 2' and 4' the active layer 3' and the ohmic contact layer 6' are formed by means of the continuous liquid phase growth process. An insulating layer 7' is formed to cover the ohmic contact layer 6', except for a stripe portion 61', so that a stripe region 31' which forms a light emitting region of the semiconductor laser is defined. On the bottom of the semiconductor substrate 1', and on the top of the insulating layer 7' and the stripe portion 61', a negative electrode 8' and a positive electrode 9' are formed, respectively. The clad layers 2' and 4' have a greater energy gap and a smaller index of refraction of light than those of the active layer 3'. In order to define the stripe region 31', the proton irradiation process or the impurity diffusion process can be used, instead of the above described process for forming the insulating layer 7'. Both end mirrors t'.sub.1 and t'.sub.2 of the semiconductor laser S' are usually formed by the cleaving process.
However, the prior art semiconductor laser of FIGS. 1A and 1B has the following disadvantages. That is, although a laser oscillation occurs in the active layer 3' due to the injected current which passes through the stripe portion 61', the Y-direction oscillation mode in the active layer 3' is determined only by the stimulated emission gain in the Y-direction due to the injected carriers and no change of the index of refraction of light exists in the Y-direction. Accordingly, the threshold current of the semiconductor laser is caused to be high and the external differential quantum efficiency of the semiconductor laser is caused to be low.
As a result, in the prior art semiconductor laser of FIGS. 1A and 1B, an intense stimulated emission occurs in the neighborhood of the center of the oscillating portion when the excitation attains high level, so that an irregular operation condition appears in the usual oscillation mode and the oscillation characteristic of the semi-conductor laser is caused to be unstable. This unstable oscillation characteristic makes the current versus optical output characteristic of the semiconductor laser in the steady state nonlinear and makes it impossible to obtain outputs which are greater than a predetermined value. In addition, the oscillation is caused to be irregular and no clean Gaussian distribution is obtained in the current versus optical output characteristic.
In connection with the semiconductor laser illustrated in FIGS. 1A and 1B, it should be noted that the distribution of the gain of the semiconductor laser corresponding to the change of the index of refraction can be expressed as indicated in FIG. 2. In FIG. 2, the line C(n) represents the distribution of the index of refraction along Y-direction of FIGS. 1A and 1B. The value of the index of refraction is n.sub.1, within the range w', which is the width of the stripe, and is n.sub.2 outside the range w'. The difference .DELTA.n which is equal to n.sub.1 -n.sub.2 is, neary zero for the prior art semiconductor laser. The curve C(g) indicates that the gain g of the semiconductor laser attains a high value within the range w'. The maximum value of the gain g is, for example, 70(cm.sup.-1) to 90(cm.sup.-1).
An example of prior art striped semiconductor light emitting devices is disclosed in the Japanese Patent Application Laid-open Publication No. 53-138689, corresponding to the United States patent application No. 794466, now U.S. Pat. No. 4,169,997.